1. Field of the Invention
The invention relates to a method for optimizing the energy balance in a forming section in a machine for the production of a fibrous web, especially a paper, cardboard or tissue web, whereby a fibrous stock suspension which is fed into the forming section through a headbox after having reached the immobility point is passed through at least two dewatering units inside one compression zone following the immobility point, to a transfer area to a following functional unit.
The invention further relates to a forming section, comprising at least one continuous wire supporting the fibrous stock suspension at least indirectly, and at least two dewatering units arranged in tandem or respectively arranged following each other in the direction of travel of the fibrous suspension inside the compression zone.
2. Description of the Related Art
The production of fibrous webs in a continuous manufacturing process occurs by forming of fibers from an aqueous suspension on a moving wire inside a forming section. Due to weight, water is removed from the suspension and from the web being formed, by means of mechanical compression, especially due to the wire tension at curved dewatering elements and with the assistance of vacuum suction through the wire. Following the dewatering process in the forming section the fibrous web is transferred to a press section in which additional water is removed from it. The web is subsequently transferred to a drying section where the drying process is completed.
Forming sections as components in a wet section of a machine for the production of fibrous webs are known in the current state of the art in a multitude of designs. Relative to their specific embodiment they are divided into single wire formers and twin wire formers. Hybrid formers represent a variation of a twin wire former with a Fourdrinier wire, whereby generally the lower wire acts as the Fourdrinier wire in the twin wire former. The essential purpose of these types of forming sections consists on one hand to achieve a targeted placement of the fibers adjacent to each other and on top of each other, as well as to achieve fiber orientation inside the fibrous suspension as desired and to further dewater the fibrous stock suspension during passage through the forming section in a way that, at the end of the forming section viewed in machine direction, a fibrous web which is characterized by an appropriately pre-defined dry content can be transferred to the subsequent processing sections, especially a press section. In order to ensure sufficient quality of the end product and to minimize reject end products the properties of the fibrous web must be continuously monitored during the production of fibrous webs, especially fibrous webs in paper or cardboard machinery. Various parameters can be used as control value in a control and/or adjustment in the production process, for example the basis weight, the water weight or also the thickness of a fibrous web in different segments inside the machine for the production of such a fibrous web. The final quality of the fibrous web is substantially influenced by processes in the forming section, for example by the formation. There are many control processes known in the current state of the art with which the quality of the fibrous web can be controlled inside the forming section through control of dewatering, revealing themselves for example in the formation, porosity, fiber orientation, the vertical sheet formation and moisture content.
An apparatus for the production of a fibrous web including a twin wire former which comprises conspiring wires which travel together over part of their rotational path by forming a so-called twin wire zone is already known from EP 1 426 488 A1. A measuring arrangement to measure one characteristic of the fibrous web in the area of, or around the twin wire zone is provided inside said apparatus, whereby the measured characteristic is fed into a control unit as an actual value and this control unit controls one production parameter for the production of the fibrous web. For example, the pressure level or vacuum in a dewatering unit inside a pre-dewatering zone is set as a control value. Based on a desired dry content of the fibrous web that was determined by the control unit, a dewatering unit located at the beginning of the pre-dewatering zone when viewed in direction of travel of the fibrous web can be used—in other words, even before the compression zone—in order to adjust the dry content of the fibrous web. The adjustment of a pre-defined formation is considered an essential objective.
A method for the operation of a forming section is already known from EP 1 454 012 B1 where the consistency of pulp inside a forming section, as well as the influence of the consistency over the formation and/or porosity of the developing fibrous web are determined and the consistency is adjusted on the basis of the quality properties of the finished fibrous web and/or though optimization of a cost function. The quality characteristic of the fibrous web is defined by its formation and/or the porosity. The cost function includes at least the costs which are conditional upon the required energy consumption and the required power supply.
A method and a system to regulate the cross profile of the stock dry weight in a fibrous web which is formed from a fibrous stock suspension in a forming section and which includes at least one continuous rotating water permeable wire is already known from EP 1 137 845 B1. Here, an actual value of the stock dry weight in the drying section is determined and based on a water weight cross profile which is determined by means of a water weight sensor inside the forming section, conclusions are made regarding an ensuing stock dry weight cross profile. The stock dry weight cross profile is regulated on the basis of the stock dry weight cross profile which was predetermined as a result of the water weight measurement.
Among other factors, all prior mentioned designs use the drainage capacity inside the forming section as the control value, whereby preferably pressures, especially partial vacuum at suction devices function as control values. In contrast EP 1 063 348 A2 offers a possibility of control/regulation of dewatering units in embodiment of forming blades.
The designs known from the current state of the art essentially meet the objective of controlling and/or of regulating the individual components of a forming section, or respectively their conspiring with each other in such a way that with regard to the result which is to be achieved relative to the ensuing material web, especially fibrous web, optimum properties of the desired kind are achieved. The cost aspect resulting from the energy balance of the entire line essentially is not considered here. As a rule, a favorable energy balance is contrary to the desired result, or in other words to achieving an appropriately high dry content after reaching the, or respectively passing through the, forming section. In many lines for example the vacuum which is to be supplied to the individual suction devices inside the forming section is pre-set to a firm value, whereby the high efficiency suction devices are often set to maximum vacuum during operation. The efficiency of dewatering is accordingly high. Due to the relative movement of the movable wire and the high-vacuum suction device, the wire—also because of high frictional forces—is subject to high wear and tear.
What is needed in the art is to develop a method for optimization of the energy balance in a forming section in such a way that even at a lower required energy supply into the forming section an optimum result regarding the required dry content is achieved, while not impairing the sheet formation. The fibrous stock suspension inside the forming section must be dewatered in an as energy saving and wear and tear preventing way as possible until the required dry content is reached.